Thank you for your question.
I am presuming (and please reply and let me know if this is not correct) that the customer feels as if they were the victim of defamation
because of your comments to the employee?
Based on these facts, I do not see a claim on their part. Defamation requires that one party make a false statement about another party to a third person or persons, resulting in damages. First, it's hard to know what exactly you said, but if the information provided was truthful (e.g. "This customer has been very difficult to work with in the past, you need to make sure you do x when you speak with them") then it cannot be defamatory. Second, the information that the customer was given was misconstrued and twisted by the employee -meaning what was said to the customer wasn't defamation, as it was the employee making the statement directly to the customer, and not to a third person about the customer. Third, even if this was an intentional or negligently made false statement about the customer, what damages did they suffer? Unless they can show they suffered some type of significant harm, there really is no claim for damages.
Now, that all said - Texas is also an at will employment state. This means that unless you have an employment contract
or union agreement, you can be terminated at any time, for any reason, with or without cause, provided you are not terminated for an unlawful reason (e.g., because of your race, age, sex, disability, religion or national origin). So, your employer could feel so threatened by the customer's comments, or even just choose to believe that what you said to the employee was impromper, or just feel like it would be in the best interest of the company - and terminate you. If you are an at will employee, they wouldn't need a reason to terminate you.